Intusoft is releasing a new version of its popular transformer
and inductor design program, Magnetics Designer.
The new version 4.0 represents a significant breakthrough in terms
of performance and new features. A major enhancement includes
the ability to handle sector winding configurations for high voltage
designs. A new "Bobbin tab" allows you to see the winding
structure (Figure 1), as well as export the structure in the DXF
file format. New 2D Finite Element Analysis (FEA) algorithms produce
magnetic field maps that describe the winding magnetic field.
The FEA solutions enable calculation of the AC losses for sector
wound geometries.

Data entry is eased with the addition of a Design Wizard (Figure
2). An expanded winding screen gives you more per winding information
such as current density and winding weight (Figure 3). Silver
and aluminum wire types are now supported, along with a core database
that contains more than 7,000 cores.

Program Overview

Magnetics Designer synthesizes all types of transformers and
inductors. Typical applications include high power transformers
for aerospace and industrial power levels, subminiature planar
magnetics and general switched mode power supply design. Virtually
any single phase, layer or sector wound inductor or transformer,
from 10Hz to over 5MHz, can be synthesized with Magnetics Designer.
Magnetics Designer is different than other FEA based analysis
tools. It actually selects the core geometry and synthesizes the
winding structure for you, making you dramatically more productive.
You do not have to draw anything or specify any physical design
characteristics other than the core family, material and type
of wire (formvar, square, Litz, foil, pcb trace) you want to use.

What Makes Magnetics Designer Unique

Magnetics Designer performs synthesis as well as analysis
of transformers and inductors.

Figure 2, The new Design Wizard helps lead you through entry
of the required parameters: Core Family, Material, Wire Type,
Operating Frequency, Voltage/Current per winding, and allowed
Temperature rise. Entering design specifications is a simple matter.

Figure 3, The expanded Transformer screen displays more
winding information. Major enhancements include the ability to
handle split bobbin configurations and more versatile specification
of the current waveform. A Fourier series is computed for the
specified pulse waveform duty cycle and rise/fall time. New Finite
Element Analysis (FEA) algorithms give you more accurate calculation
of the AC losses.

Classical design methods rely on home-grown software programs
that are patched together, or "build and test" iterations
that are exercised until the appropriate behavior of the prototype
component is achieved. Magnetics Designer eliminates these costly
design methodologies by accurately synthesizing and analyzing
the magnetic structure for you.

Magnetics Designer achieves significant gains in productivity
by combining advanced analytical algorithms and 2D FEA techniques
in a single, easy-to-use software tool. Initially, multi-parameter
optimizations are employed to handle the design variable trade-offs
(about 10,000 optimization passes per design). Finite element
calculations are performed after the speedier analytical optimization
algorithms. Tools which use FEA as the primary solution path can't
perform the design variable trade-offs efficiently due to long
run time constraints. Thus, the user is forced to choose the specific
core geometry and winding structure. This is a major drawback.
Magnetics Designer, on the other hand, does this for you.

Since the multi-parameter optimization algorithms are fast,
the user can compare designs quickly and easily. The user can
change more than 20 parameters such as allowed temperature rise,
allowed window fill, number of turns, wire gauge and type, bobbin
margins, number of parallel strands, gap length, layer insulation
thickness, wrapper thickness, and end margin lengths. The program
allows the user to split, move, and interleave windings and set
variable size sectors.

The result is a tool that generates an optimized design with
accurate calculations of all key performance characteristics from
a simple list of electrical, mechanical, and thermal specifications.
Designers can consider various geometry, material and winding
strategies without actually building the component, thereby ensuring
a low cost, efficient design.

The "Power Specialist's App Note Book" contains over
35 technical articles on power supply design and power electronics
modeling. The papers cover a wide range of topics including: SMPS
design, magnetics modeling, signal generator modeling, power semiconductor
modeling, and power supply testing. Many useful examples and modeling
tips are included, as well as detailed discussions of key power
supply design issues.

The ICAP/4 simulation system contains many unique features.
If you have used, or are using other SPICE packages, you may not
be familiar with them. In this installment we discuss Automated
Measurements.

The traditional post analysis methods for SPICE output data
are analogous to using an oscilloscope. You select and display
a waveform from the list of saved data, move cursors on the waveform
and make your measurement (peak-peak, max, min, etc.) This process,
while simple, is too time consuming when dozens of waveforms must
be analyzed and hundreds of measurements must be made. This is
the case when performing design validation, failure analysis or
test program development.

Fortunately, ICAP/4 includes a new paradigm for waveform analysis.
It's called Automated Measure-ments. The process is simple. Using
a series of "Wizard" dialogs (Figure 4), you setup the
desired measurements. Virtually any circuit quantity can be recorded,
including DC, rise/fall time, prop. delay and triggered measurements
on voltages, currents, or power dissipations. You do not have
to write any scripts or do any programming to setup the measurements.

The voltage at any node, and the current or
power of any device can be selected.

Figure 4, ICAP/4 allows you to setup several circuit variations
with different topologies and part values (left). You can then
assign as many automatic measurements as you want using a graphical
Wizard approach (right).

Figure 5, The measurements are recorded automatically after
the simulation is complete. The nominal measurement performance,
shown above, can be viewed after setting the min/max limit boundaries
for each measurement.

The measurements are automatically performed by IsSpice4 each
time a simulation is run, and the results are compiled into a
report (Figure 5). You can set limits or stress alarms for each
measurement. The report will tell you how any circuit change (stimulus,
part value, model parameter, etc.) affects your design (Figure
6). The best part is that the data gathering, calculation of the
measured quantity, and report generation are completely automatic
and transparent to the user. No user interaction is needed to
process the measurements!

Why is this feature important? It gives you a general
data analysis framework that is far more productive than the traditional
methods of waveform analysis.

Figure 6, The measurements (Measured column) are recorded
automatically after the simulation is complete, and are compared
with the nominal values. Variations against the test limits can
be easily seen using the Meter column.

Inverters, as defined here, convert DC power into AC power.
They are widely used in single and three phase industrial applications.
To limit heat dissipation, efficiencies greater than 90% are of
primary interest, especially in the kilowatt range. Power transistors
therefore, operate as switches, generating a square wave voltage
at the output terminals as shown in the 400Hz-Inverter in Figure
7. Many types of equipment operate satisfactorily on a square
wave supply voltage, but in some instances, a filter at the output
is necessary (as shown in Figure 9) to suppress objectionable
harmonics.

The power MOSFETs work in a push-pull mode. No net direct current
flows through the center-tapped transformer, thereby reducing
its size and weight. In a similar manner, all even-order harmonics
are cancelled by symmetry (Figure 8). Stabilization of the output
voltage is accomplished by a feedback loop that regulates the
pulse width of the gate waveforms for the power MOSFETs. Figure
8 shows that with increasing DC-supply voltage, V0, the pulse
width of output voltage decreases accordingly, keeping its RMS
value at a constant level.

Figure 7, 400Hz push-pull inverter. The transformer was
designed and modeled with Magnetics Designer. VO supplies the
DC input voltage.

Figure 8, Plots of the output and clock signals (top graph),
the RMS value of the output (lower plot, waveform 1), and the
output frequency response. As the FFT of the output shows, even
harmonics are suppressed.

The transformer design and SPICE model was created with Magnetics Designer. The following specifications
were entered:

Magnetics Designer Input and
Output Parameters for the 400Hz Transformer

Input Specifications

Synthesized
Design Results

Operating Frequency:

440Hz

Core Loss

1.959 (Watts)

Core Family/Material

EI Lamination 6mil Square

Copper Loss

5.464 (Watts)

Temperature rise above ambient

35 degrees C

Weight

2.014

(Core Weight: 1.228, Copper Weight: 0.7856,
in pounds)

#1p

#2s

#3p

#1p

#2s

#3p

Voltage per winding

62

880

62

Turns of wire

57

810

57

AC RMS current per winding

3.4

350m

3.4

Wire Gauge

19

29

19

The digital gates were modeled at the transistor level. They
could have been modeled with digital gates, but the close ties
to the analog elements would have required several A-D and D-A
bridges (8 total) which in themselves can increase circuit complexity.
In the end, the complexity is approximately equal. ICAP/4 includes
digital gates modeled at the transistor level and at the gate
level, so users can have a choice of which approach to take depending
upon the ratio of digital to analog circuit content.

Figure 9, A 400Hz SCR inverter.

400Hz SCR Inverter

One of the major applications for SCRs is converting DC to
AC power in the kilowatt range. Again, high efficiency is an important
feature. For supply voltages above 24Vdc, values greater than
90% may be obtained. To avoid unilateral saturation of the output
transformer and to make use of the full dynamic range of the magnetic
material, a parallel inverter configuration is used to deliver
symmetrical square waves under all load conditions (Figure 9).
All odd harmonics are cancelled by the push-pull mode of operation.
Feedback diodes prevent the voltage across the primary windings
from exceeding the supply voltage.

Because SCRs cannot be turned off by their gate signal, a return
to the blocking state has to be forced by reversing the anode
voltage. Commutation is initiated by turning on the nonconducting
SCR. For a small period of time, both SCRs are conducting. To
avoid a short circuit, a small inductance, L7, is inserted. It
operates as a choke by taking up the battery voltage. Finally,
the conducting SCR is turned off by the negative charge that is
stored in commutating capacitor C1. Values of C1 and L7 depend
upon the supply voltage, the SCR recovery time (10-20µs)
and the current to be commutated.

To get an undistorted sine wave output, a wave shaping filter
has been added to eliminate all harmonics in the square wave voltage.
The transfer function of the filter is independent of the load
at the fundamental frequency.

For the novice, and sometimes well seasoned SPICE users, analog
simulation can present many pitfalls. In this section, we try
to address some of them and provide advice and insight on how
to avoid problem areas.

Ideal Elements: Parasitics

Characterizing the circuit, also known as modeling, is one
of the most important aspects of simulation. Modeling issues pervade
the entire simulation process. Therefore, it is not surprising
to find pitfalls with even the most basic passive elements. In
most schematic packages, placing a resistor, capacitor or inductor
inserts an ideal version of the element. This means that there
are no parasitics included.

Fortunately, such parasitics aren't usually critical to the
design performance. Well, at least you hope you've designed your
circuit to be parasitic intolerant. But as far as the simulation
accuracy is concerned, it's up to you to determine where you want
to add complexity in terms of element parasitics.

R-L-C Component Frequency Spectrum Limitations

Electronic circuits are always modeled over a finite range
of the electromagnetic frequency spectrum. There is really no
need to describe operation of electrical components from DC through
RF, to microwave and on to the optical, x-ray and gamma ray spectrum.
Not only would the model be complex, it would also be inaccurate
and would provide unnecessary information.

The nodal system of equations which SPICE solves are valid
only when the circuit elements are small compared to the wavelength
of the highest frequency of interest; this limits the high frequencies
below the optical band. Even with this limitation, the useful
frequency range runs from milli-hertz to many gigahertz, over
15 orders of magnitude. The reactance chart of Figure 10 shows
the expected range of parasitic inductance and capacitance over
this range. The darkly shaded region represents the values of
impedances that are realistically achieved with common R-L-C components
and printed circuit board technology.

The lightly shaded region of impedances can be viewed as a
transition region where parasitics become increasingly important.
The boundary between the lightly shaded region and the unshaded
region represents the smallest capacitance or inductance parasitic
value, and therefore values in the unshaded area are unrealistic
for single discrete components. At the high frequency end, this
suggests the use of smaller geometry microwave integrated circuits,
while extending the impedance range at lower frequencies requires
larger geometries than are ordinarily found in PC Card technology.

The modeling additions for various components are shown in
the pictorial inlays. First, resistors are basically defined at
DC, and turn into effective capacitors or inductors, their impedance
converging to that of free space divided by the square root of
the dielectric constant, something in the neighborhood of 125
Ohms for PC cards. Similarly, capacitor and inductor impedances
funnel toward the impedance of the propagating medium at high
frequencies, and become resistive as the frequency approaches
DC.

SPICE built-in models provide reasonable first order approximations
for circuit behavior. Unfortunately, most circuits must be designed
to be tolerant of second order effects, and sometimes to compensate
in order to achieve a desired performance level. Most frequently,
the parasitic and second order effects are related to changes
in frequency.

The Intusoft
model libraries and IsSpice4 can help determine which parasitics,
if any, will affect your design performance.

Figure 11, Passive component subcircuit netlists including
1st order parasitics. The parasitic values are passed into the
subcircuit at runtime or left at their default values.

Subcircuit based models can hide the parasitics, while parameter
passing gives you the freedom to specify their values. The nonideal
passive element subcircuits, shown in Figure 11, are included
in all ICAP/4 packages. Note: Sophisticated
models for tantalum and ceramic capacitors were covered in newsletters
#44 (Nov. '95) and #45 (Feb '96) (Past issues of the newsletter
are available from Intusoft).